A first prior art method for measuring a temperature of a bipolar transistor uses a measured value of a base to emitter voltage (Vbe) of the bipolar transistor. This first prior art method is referred to as the Vbe method. A second prior art method uses a measured value of a change in the base to emitter voltage (ΔVbe) of the bipolar transistor between two values of injected bias current to measure the temperature of the bipolar transistor. This second prior art method is referred to as the ΔVbe method.
Many remote temperature sensors that are now marketed use the ΔVbe method to remotely sense the temperature of the bipolar transistor. These prior art sensing devices determine the values of two Vbe measurements serially. That is, a first measurement of Vbe is made and then later a second measurement of Vbe is made. Then one of the two measurements is subtracted from the other to obtain the ΔVbe value which may be translated into an absolute temperature value for the bipolar transistor.
The temperature characteristic of the Vbe at a fixed value of current may be process dependent or device dependent. This means that the Vbe voltage measured across a bipolar transistor at a particular absolute temperature may vary between bipolar transistors made in different fabrication processes or even between bipolar transistors made in the same fabrication process. However, the Vbe voltage can be accurately used to monitor the relative change of temperature in a small temperature range regardless of the process/device sensitivity. In prior art remote temperature sensing, both the Vbe at low current values and the Vbe at high current values are measured in order to calculate the absolute temperature of the bipolar transistor.